What are high-frequency/low-frequency signals in an image?

Question

I often encountered the term high-frequency/low-frequency signals in an image. And relevantly, high-pass filters for sharpening. I have a hard time relating the word frequency with 2d images. An image is simply a 2-d array of 3 values (r,g,b). What does frequency mean in this context? And how does a high-pass filter become sharpening the image?

Answer 1

It is related to fourier transform (AFAIK, t in the formula is x,y of pixel coordinate).
Roughly speaking :-

high-frequency = high-signal when convoluted with high-frequency function      
              && relatively low-signal when convoluted with low-frequency function

low-frequency = vice-versa

I am not so strong in this area.
To avoid stealing their contents, here are all good links.
They should contain information more than you need :-

• https://photo.stackexchange.com/questions/40401/what-does-frequency-mean-in-an-image (image & code)
• https://users.cs.cf.ac.uk/Dave.Marshall/Multimedia/node223.html (very short)
• https://dsp.stackexchange.com/questions/1637/what-does-frequency-domain-denote-in-case-of-images (simple image and show fourier transform)